As the energy efficiency and environmental protection become increasingly important, the customers expect more and more on the standby efficiency of power supply products. The power supply products today are required to satisfy the Green Energy Standard, for example, BLUE ANGEL, ENERGY STAR and ENERGY 2000 and so on. Thus, many new technologies, like pulse skipping mode, burst mode and off time modulation, are developed to meet the requirement. All theses technologies are applied to control the switching mode power supply working at lower frequency when there is no load (the load current is zero or is almost zero, or the power supply is open) or when the load is very light (the load current is small).
In order to reduce the frequency of the switching mode power supply when there is no load or when the load is light, load detecting methods are needed. The conventional way of detecting the load is to couple a resistor in series to the load, and then compare the voltage across the resistor with a preset threshold to determine whether a no-load state happens. However, the resistance of the resistor is difficult to choose. Because if the resistance is large, the power consumption of the resistor is high when the load is heavy; and if the resistance is small, the voltage across the resistor is too small to detect and may be easily disturbed when the load is light.
The present disclosure pertains to detecting the load of the switching mode power supply, which may be used for frequency control and other power supply applications.